1. Field of the Invention
The present invention relates to the manufacture of a solid state electrochemical cell, and more particularly, a solid state cell having a lithium anode and a cathode composition/current collector element.
2. Description of the Prior Art
Solid state electrochemical devices are the subject of intense investigation and development. They are described extensively in the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 to Armand; 4,589,197 to North; 4,547,440 to Hooper et al.; and 4,228,226 to Christiansen. These cells are typically constructed of an alkali metal foil anode, typically a lithium foil, an ionically conducting polymeric electrolyte, a finely divided transition metal oxide cathode, and a cathode current collector which is attached to the face of the cathode not contacting the electrolyte. The current collector usually employed is a sheet of metal foil such as aluminum, nickel, or stainless steel.
Although the above described cells have presented a viable option to older, more traditional secondary type discharge cells, the rechargeability and impedance of the cells have not achieved optimal performance. Part of the problem lies in the failure of the cathode material to form a good electric contact with the current collector. Failure of the cathode material making a good electrical contact with the current collector leads to an overall increase in cell impedance. This in turn, makes it difficult to recharge the cell.
In theory, optimal performance occurs if the cathode material is in intimate contact with the cathode current collector, and wherein the cathode current collector has a high surface area to enable uniform contact between the cathode material and the collector. Attempts have been made in the art to increase both the adherence of the cathode material to the current collector, and to increase the surface area of the current collector. However, no such attempts have been made in the field of solid state alkali metal anode cells.
For example, U.S. Pat. Nos. 4,751,157 and 4,751,158 to Uchiyama et al. disclose cathode materials for use in lithium electrochemical cells. The cathode material comprises a mixed metal oxide as an active material, along with a conductive diluent and a binder which is pressed into electrodes on a nickel screen and sintered under vacuum. The cathode materials are used in cells which contain a liquid electrolyte, and more particularly, those which contain LiAsF.sub.6 in an aprotic solvent, such as methyl formate.
U.S. Pat. No. 4,416,915 to Palmer et al. discloses a chalcogenide cathode made by applying a slurry of a mixture containing at least one intercalatable layered transition metal chalcogenide cathode active material, a conductivity enhancing agent and a binding agent in a vehicle to a high porosity current collector substrate, for example, foamed metals and glasses which are 97% to 90% porous with 10 to 1000 pores per square inch and adhering the cathode material to the substrate. The cathode material is utilized in a non-aqueous lithium cell having an electrolyte comprising an electrolyte-solvent mixture.
U.S. Pat. No. 4,560,632 to Alberto discloses a molded porous cathode collector for use in non-aqueous cells. The collector includes a particulate carbonaceous conductive material bonded with a suitable binder, and having on its surface a coating of a vinyl polymer film to improve its mechanical strength and handling characteristics. The cathode collector is used in association with liquid cathode materials.
In the field of solid state lithium cells, U.S. Pat. No. 4,735,875 to Anderman et al. discloses a cell wherein a cathode material which takes the form of a microporous sheet containing polyethylene, an electrically conductive and electrochemically active particulate material and a plasticizer is laminated to a current collector such as a screen, grid, expanded metal, woven or non-woven fabric formed from efficient electron conductive materials such as carbon, or metal such as copper, aluminum, nickel, steel, lead or iron. Despite the increased surface area of the cathode collector, the Anderman et al. cell does not optimize the adherence of a cathode material to the collector as the cathode material does not necessarily interpenetrate the pores and the collector substrate.
Accordingly, there exists a need in the art for a solid state alkali metal cell wherein a highly uniform electrical contact between the cathode material and cathode current collector is maintained during operation and recharging of the cell.